std::vector with negative subscript?

[lucky6969b]

My application I am developing is purely geometric.
So sometimes I need to index an array with a negative subscript.
For example, the extent of a scene may span from a negative range to a positive range.
That forms a grid. But std::vector won't allow me to use a negative subscript.
Are there any workaround this?
Thanks
Jack

[laserlight]

You cannot access an array with a negative subscript because that would be accessing the array out of bounds. What you can do is to have a pointer point to somewhere in the middle of the array, upon which a negative subscript could access an element that actually exists.

In the case of a std::vector, perhaps you could write a function to translate the given subscript to the actual index of the element in the vector. The pointer trick could work too.

[VictorN]

Well, you could use a plain C array instead.
Or implement the additional vector to recalculate the main vector indexes.

[lucky6969b]

I finally resort to a O Log (N) solution, by specifying a key like

stringstream oss;
oss << x << ":" << z;
grid[oss.str()] = node;

It gets slower, but I think it's more robust.
Thanks
Jack

[OReubens]

string based indexes... how... horribly inefficient and non-robust (what do you expect to happen when the input string doesn't look like a "X:Z" but is "hello world" ?

[2kaud]

But std::vector won't allow me to use a negative subscript.

As vector uses random access iterators, you can add or subtract a number to a vector iterator if the resulting iterator is not out of bounds. See http://www.cplusplus.com/reference/iterator/

So if you obtain a vector iterator that points somewhere within the vector then a number can be added or subtracted from this iterator.

[GCDEF]

If the boundaries of your range are known, such as -10,000 - 10,000, you could use a 20,000 element vector and just add 10,000 to your index.

[D_Drmmr]

My application I am developing is purely geometric.
So sometimes I need to index an array with a negative subscript.
For example, the extent of a scene may span from a negative range to a positive range.
That forms a grid. But std::vector won't allow me to use a negative subscript.
Are there any workaround this?
Thanks
Jack

You can use Boost.MultiArray, which allows setting an 'index base' to change the index of the first element in the array.